Chip joining by induction heating

ABSTRACT

Methods and apparatus for joining a chip with a substrate. The chip is moved by with a pick-and-place machine from a first location to a second location proximate to the substrate over a first time. In response to moving the chip in a motion path from the first location to the second location, a plurality of solder bumps carried on the chip are liquefied over a second time that is less than the first time. While the solder bumps are liquefied, the chip is placed by the pick-and-place machine onto the substrate.

BACKGROUND

The invention relates generally to semiconductor packaging and, inparticular, to systems and methods for joining a chip to a substrate.

A die or chip includes integrated circuits formed by front-end-of-lineprocessing using the semiconductor material of a wafer, a localinterconnect level formed by middle-end-of-line processing, and stackedmetallization levels of an interconnect structure formed by back-end-ofline processing. After the wafer is diced, each chip may be joined witha substrate using a controlled collapse chip connection or flip chipprocess. In a flip chip process, reflowed solder bumps providemechanical and electrical connections between pads in the topmetallization level of the chip and a complementary set of pads on thesubstrate. The solder bumps can be formed on the pads of the chip usingany number of techniques, including electroplating, evaporation,printing, and direct placement. Reflow of the solder bumps establishessolder joints physically and electrically connecting the pads on thechip with the complementary set of pads on the substrate.

Systems and methods for joining a chip to a substrate are needed thatimprove on existing joining systems and methods.

SUMMARY

In an embodiment of the invention, a method is provided for joining achip with a substrate. The method includes moving the chip with apick-and-place machine in a motion path from a first location to asecond location proximate to the substrate over a first time. Inresponse to moving the chip from the first location to the secondlocation, a plurality of solder bumps carried on the chip are liquefiedover a second time that is less than the first time. While the solderbumps are liquefied, the chip is placed by the pick-and-place machineonto the substrate.

In an embodiment of the invention, an apparatus is provided for joininga chip with a substrate. The apparatus comprises a pick-and-placemachine and an induction heater. The pick-and-place machine includes apick-up head configured to releasably hold the chip and a robotic armconfigured to move the pick-up head. The induction heater is located ina motion path of the chip to the substrate and is configured to heatsolder bumps on the chip by electromagnetic induction while the chip isreleasably held by the pick-up head.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention.

FIG. 1 is a diagrammatic view of an induction heating system inaccordance with an embodiment of the invention.

FIG. 2 is a block diagram of the controller in FIG. 1.

FIG. 3 is a cross-sectional view of the solder bumps on a chip whilebeing heated with the induction heating system of FIGS. 1 and 2 prior tocontact of the solder bumps with pads on a substrate.

FIG. 3A is a cross-sectional view of a portion of FIG. 3 in which a padon the substrate is depicted.

FIG. 4 is a cross-sectional view similar to FIG. 3 after the moltensolder bumps have solidified to form solder joints coupling the pads onthe chip with the pads on the substrate.

FIG. 5 is a flowchart that illustrates a sequence of operations that maybe performed by the induction heating system.

FIG. 6 is a diagrammatic view of an induction heating system inaccordance with an alternative embodiment of the invention.

DETAILED DESCRIPTION

With reference to FIGS. 1-4 and in accordance with an embodiment of theinvention, a system 10 configured to use induction heating to formsolder joints 16 between a chip 12 and a substrate 14 is shown. Thesystem 10 includes a controller 18 with at least one processor 20including at least one hardware-based microprocessor and a memory 22coupled to the at least one processor 20. The memory 22 may representthe random access memory (RAM) devices comprising the main storage ofcontroller 18, as well as any supplemental levels of memory, e.g., cachememories, non-volatile or backup memories (e.g., programmable or flashmemories), read-only memories, etc. In addition, memory 22 may beconsidered to include memory storage physically located elsewhere in thecontroller 18, e.g., any cache memory in a microprocessor, as well asany storage capacity used as a virtual memory, e.g., as stored on a massstorage device or on another computer coupled to the controller 18.

For interfacing with a user or operator, the controller 18 may include auser interface 24 incorporating one or more user input/output devices,e.g., a keyboard, a pointing device, a display, a printer, etc.Otherwise, input may be received via another computer or terminal over anetwork interface 26 coupled to a communication network. The controller18 also may be in communication with one or more mass storage devices,which may be, for example, internal hard disk storage devices, externalhard disk storage devices, external databases, storage area networkdevices, etc.

The controller 18 typically operates under the control of an operatingsystem 28 and executes or otherwise relies upon various computersoftware applications, components, programs, objects, modules, engines,data structures, etc., including for example, a placement module 30 anda heating module 32. In general, the placement module 30 may beconfigured to control the operation of a pick-and-place tool or machine40 when its instructions are executed by the at least one processor 20of the controller 18 in order to pick up each chip 12 and transfer eachchip 12 from a storage location 44 to the substrate 14. The storagelocation 44 may comprise a wafer or may comprise a chip bank. Theheating module 32 may be configured to control the operation of a powersupply 46 when its instructions are executed by the at least oneprocessor 20 of the controller 18 in order to power an induction heater48 during the chip attach process. Moreover, various applications,components, programs, objects, modules, etc. may also execute on one ormore processors in another computer coupled to the controller 18 via thecommunication network, e.g., in a distributed or client-server computingenvironment, whereby the processing required to implement the functionsof a computer program may be allocated to multiple computers over anetwork. The memory 22 may store one or more data structures including,for example, a database 34 configured with records 36 to store datarelating to the process (e.g., the position of each chip 12 at thestorage location 44, instructions for the operation of thepick-and-place machine 40, control settings for the power supply 46,etc.

The pick-and-place machine 40, which is in communication with thecontroller 18 through a pick-and-place machine interface 29, includes arobotic arm 41 and at least one pick-up head 42. The pick-up head 42 isequipped with an acquisition device, such as pneumatic suction cups,capable of temporarily and releasably holding the chip 12. The roboticarm 41 is configured to move the pick-up head 42 over a motion path 43,which is diagrammatically indicated by the single-headed arrows inFIG. 1. The motion path 43 may originate at the storage location 44where each chip 12 is picked by the pick-up head 42 and terminate atanother location in the form of one of the substrates 14. The motion ofthe pick-up head 42 may be unbroken and continuous over the motion path43.

The robotic arm 41 may be a programmable mechanical arm with linksconnected by joints allowing rotational motion and/or translationaldisplacement of the pick-up head 42. The robotic arm 41 may be, forexample, a three-axis R-Theta robot arm or a selectively compliantarticulated robot arm (SCARA). The robotic arm 41 is configured tomanipulate and accurately position the pick-up head 42 and to move thepick-up head 42 in response to program code executed by the at least oneprocessor 20, user interaction with the user interface 24, and/or otherinstructions or input received by the at least one processor 20.

The substrates 14 may be located on a conveyor 60. The operation of theconveyor 60 may also be controlled by the controller 18. However, thesubstrates 14 may be held and/or moved in a different manner such as adevice table or other platform. Each substrate 14 is held stationary bythe conveyor 60 at the time of the physical contact between the chip 12and substrate 14 during the joining process.

A heater 58 may be positioned relative to the conveyor 60 so that eachsubstrate 14 can be heated, prior to chip-joining, to a temperature thatapproximates the temperature of the substrates 14 during use in an endproduct (e.g., 60° C. to 100° C.). The heater 58 may comprise a radiantheater configured to generate radiant energy, a lamp configured togenerate light, or an air heater configured to generate warm air. Theheater 58 may be configured to heat the substrates 14 from the frontsideor from the backside.

The induction heater 48 is located proximate to the substrates 14. Theinduction heater 48 may be a freestanding structure or may includeadditional support structure to provide support. In a representativeembodiment, the induction heater 48 may be a coil consisting of multipleturns of a conductor wound in helical configuration, and thetime-varying magnetic field is formed in and around the turns of thecoil as described by the Biot-Savart law.

The induction heater 48 is coupled with the power supply 46, which is incommunication with the controller 18 through a power supply interface27. Electrical power is supplied from the power supply 46 to theinduction heater 48 in response to program code executed by the at leastone processor 20, user interaction with the user interface 24, and/orother instructions or input received by the at least one processor 20.The power supply 46 may supply high-frequency alternating current to theinduction heater 48. In one embodiment, the alternating current may besupplied to the induction heater 48 at a radio frequency, such as afrequency of 13.6 MHz.

The time variation in the high-frequency alternating current produces atime-varying magnetic field at the induction heater 48. The strength ofthe magnetic field generated by the induction heater 48 may be on theorder of one (1) Tesla, but other field strengths may be applicable. Ina representative embodiment, a coil operating as the induction heater 48may have a diameter on the order of two (2) inches to three (3) incheswith four (4) to five (5) turns and may be cooled by a cooling mediumflowing through a hollow interior of the coil.

The induction heater 48 is dimensioned such that the chip 12 can bepositioned while held by the pick-up head 42 at a location proximate tothe induction heater 48 and the solder bumps 50 on the chip 12influenced by the time-varying magnetic field. For example, a coilserving as the induction heater 48 may be dimensioned with an innerdiameter that is large enough to receive the chip 12 so that the chip 12can be moved axially by the pick-up head 42 through the interior of thecoil turns. The induction heater 48, which may be located in proximityto the substrate 14 at all times of the pick and place operation andjoining operation, may be fixed at a static or fixed location and bestationary relative to the pick-up head 42 of the pick-and-place machine40. The pick-up head 42 of the pick-and-place machine 40 provides themotion of each chip 12 over the motion path 43 toward a target locationgiven by the substrate 14 that is the intended recipient of the chip 12and that is aligned with the induction heater 48. Each substrate 14 maybe sequentially moved by the conveyor 60 into location proximate to theinduction heater 48 when the particular substrate 14 is the targeted onefor chip attachment.

The induction heater 48 may be located at a different position in themotion path 43 for the pick-up head 42 and its positioning is notlimited to being located directly above the substrate 14 on the conveyor60. The proximity of the induction heater 48 to the substrate 14 can berelaxed so long as the induction heater 48 is located in the motion path43. For example, the induction heater 48 may be located with higherproximity relative to the storage location 44 for the chips 12 and lowerproximity relative to the substrates 14 and conveyor 60. The robotic arm41 can move the pick-up head 42 and chip 12 relative to the inductionheater 48 to liquefy the solder bumps 50 on the chip 12 after movementover a segment of the motion path 43 and then, after heating withelectromagnetic induction, continue to move the pick-up head 42 and chip12 over the remainder of the motion path 43 to the substrate 14. Thetime required for movement of the chip 12 held by the pick-up head 42over the remainder of the motion path 43, after heating, to place thechip 12 onto the substrate 14 is chosen such that the solder bumps 50remain liquefied and in the liquid phase at the time of placement.

The chips 12 at the storage location 44 are formed by processing a waferwith front-end-of-line processes to fabricate one or more integratedcircuits on each chip 12. The integrated circuits on each chip 12 areconnected with each other and the external environment by aninterconnect structure fabricated using middle-end-of line andback-end-of-line processes. The individual chips 12 may be singulatedfrom the wafer by mechanical sawing, by scribing and breaking, by lasercutting, or by a different technique.

As best shown in FIG. 3, the chip 12 includes pads 52 formed in atopmost metallization level of the interconnect structure and solderbumps 50 that are formed on or placed onto the pads 52. The substrate 14includes complementary pads 54 that participate in the chip joiningprocess. When processed by the induction heater 48 of the system 10, thesolder bumps 50 (FIG. 3) are transformed by heating with electromagneticinduction from a solid phase to a liquid phase. After placement toestablish contact between the liquefied solder bumps 50 and the pads 54on the substrate 14, the liquefied solder bumps 50 subsequently coolfrom the liquid phase back to the solid phase to form the solder joints16 (FIG. 4).

The solder joints 16 are configured to permanently attach the chip 12 tothe substrate 14. As best shown in FIG. 4, each solder joint 16mechanically couples one of the chip pads 52 with one of the substratepads 54. The solder joints 16 and pads 52, 54 provide electricalpathways for transferring signals between the integrated circuits of thechip 12 and an external device, such as a computing system, andelectrical pathways for powering and grounding the integrated circuitsof the chip 12.

The solder bumps 50 may be separately formed and transferred to the pads52 by a controlled collapse chip connection (C4) technology. To thatend, an area array of injection-molded solder bumps 50 is formed usingbulk solder injected into cavities in a mold plate. The plate cavitiesmatch the locations of chip pads 52. The solder bumps 50 populating thecavities are transferred to each chip 12 by precisely aligning the bumpswith the chip pads 52 and executing a reflow transfer. Alternatively,each solder bump 50 may be formed by electroplating to its chip pad 52using an appropriate plating solution, anodes, and direct currentsupplied to the anodes.

The substrate 14 may be comprised of an organic material, such as apolymer or plastic that may optionally be reinforced. Alternatively, thesubstrate 14 may be comprised of a ceramic material. In an embodiment,the substrate 14 may be a leaded or leadless chip carrier that iscomprised of either an organic material or a ceramic material.

The solder bumps 50 may be comprised of solder having a lead-free(Pb-free) composition, a eutectic tin/lead (Sn/Pb) composition, a highlead (Pb) composition, etc. The solder bumps 50 do not have to becomprised of a specially engineered solder material because theelectromagnetic properties of existing solder materials may be adequatefor coupling with the time-varying magnetic field to provide the eddycurrent heating causing the transition from a solid phase to a liquidphase.

The pads 52, 54 may be comprised primarily of aluminum (Al) or copper(Cu), and may further include one or more layers of other materials,such as titanium tungsten (TiW), nickel (Ni), etc., comprising underbump metallurgy. In particular, the pads 54 on the substrate 14 mayinclude a layer 56 comprised of a material, such as Ni, that is heatedby the induction heater 48. The localized heating of the layer 56 maywarm the pad 54 to a given temperature, such as a given temperature thatis less than the melting point of the solder comprising the solder bumps50, and without significant warming of the remainder of the substrate14.

FIG. 5 provides a flowchart that illustrates a sequence of operationsthat may be performed by the system 10 of FIG. 1 consistent withembodiments of the invention. The controller 18 may operate thepick-and-place machine 40 to move its pick-up head 42 to the storagelocation 44 and pick up the chip 12 at the storage location 44 (block100). The controller 18 may operate the pick-and-place machine 40 tomove its pick-up head 42, with the chip 12 secured to the pick-up head42, in the motion path 43 toward the substrates 14 on the conveyor 60(block 105). The motion path may be continuous and unbroken. Thecontroller 18 may operate the conveyor 60 to align the substrate 14 thatis the intended recipient of the chip 12 with the induction heater 48(block 110). The solder bumps 50, and the chip 12, may be at roomtemperature while located at the storage location 44 and prior toheating by the induction heater 48.

During movement of the chip 12 from the storage location 44 to thesubstrate 14, the solid solder bumps 50 are melted or liquefied byelectromagnetic induction using the induction heater 48 in a time thatis less than the time required to complete the movement over the motionpath 43 (block 115). Specifically, the controller 18 operates the powersupply 46 to supply a time-varying current in the coil of the inductionheater 48, which in turn produces a time-varying magnetic field in thespace within and about the coil of the induction heater 48. For example,the power supply 46 may be triggered by the controller 18 to energizethe induction heater 48 immediately before the chip 12 enters the coil.As the pick-up head 42 moves the chip 12 axially through the coil of theinduction heater 48, the time variation of the magnetic field induceseddy currents in the electrically-conducting material comprising thesolder bumps 50 that, due to the electrical resistance of theelectrically-conducting material, heat by electromagnetic induction. Thesolder bumps 50 on the chip 12 may be simultaneously heated by theinduction heater 48 to a temperature greater than the melting point ofthe constituent solder material. While in the liquid state, the solderbumps 50 remain coupled with the pads 52 on the moving chip 12.

The heating time and parameters for the high-frequency alternatingcurrent supplied to the induction heater 48 are selected to melt orliquefy the solder bumps 50. For example, the solder bumps 50 may beheated to a temperature that is greater than a melting point of thesolder and to an absolute temperature that may be contingent upon soldercomposition among other factors, but that is typically in a range of175° C. to 350° C. In one embodiment, the heating time required toliquefy (i.e., melt) the solid solder bumps 50 may be on the order oftens of microseconds. In another embodiment, the heating time requiredto liquefy the solid solder bumps 50 may range from twenty (20)microseconds to two hundred fifty (250) microseconds. In such timedomains, the induction heating operation can be performed while placingthe chip 12 on the substrate 14 with negligible additional impact oncapacity and cycle time. During the operation of the induction heater48, the chip 12 and the substrate 14 may not be heated.

The substrates 14 may be at room temperature or may be heated by theheater 58 prior to the chip 12 being placed on and joined to thesubstrate 14. Alternatively, the induction heater 48 may be used tolocally heat the pads 54 at the time of joining by coupling of thetime-varying magnetic field with the layer 56.

While the solder bumps 50 are in the liquid phase, the controller 18 mayoperate the pick and place machine to move its pick-up head 42 to placethe chip 12 on the substrate 14 (block 120). For example, each solderbump 50 on the chip 12 may be placed into contact with one of thesubstrate pads 54 while the solder bumps 50 are liquid. The moltensolder bumps 50 may react with one or more materials of the substratepad 54 to provide a metallurgical bond. Either before or after contactbetween the molten solder bumps 50 and the substrate pads 54, thecontroller 18 may operate the power supply 46 to discontinue thetime-varying current in the coil of the induction heater 48, and therebycease the application of the time-varying magnetic field. With thesource of heating eliminated, the solder bumps 50 can rapidly cool belowtheir melting point and return to a solid phase to form the solderjoints 16.

The solder joints 16 (FIG. 4) mechanically and electrically attach thechip 12 to the substrate 14 by bonding with the substrate pads 54 (block125). Each solder joint 16 permanently attaches one of the chip pads 52with one of the substrate pads 54. The process of FIG. 5 may be repeateduntil the supply of chips 12 and/or the supply of substrates 14 isexhausted and requires replenishment.

Forming the solder joints 16 with induction heating may reduce theimpact of differences between the coefficients of thermal expansionbetween the materials of the chip 12 and substrate 14 during chipjoining. In instances in which the chip 12 is primarily comprised ofsilicon and the substrate 14 is primarily comprised of an organicmaterial (e.g., a plastic or polymer), the respective coefficients ofthermal expansion may be highly mismatched and the substrate 14 mayexpand to a greater degree than the chip 12. The heating of the chip 12and/or the substrate 14, if any, is to an extent that negates the impactof the mismatched coefficients of thermal expansion. As a result, thesolder joints 16 may be more reliably produced and are less likely tode-bond from the substrate pads 54, and the substrate 14 may be lesslikely to warp or bow in response to the joining process so that theplanarity of the substrate 14 is preserved. The use of induction heatingduring chip joining may also reduce or eliminate residual strain in thejoined chip 12 and substrate 14 and/or reduce or eliminate misalignmentbetween the solder bumps 50 and substrate pads 54 that may ordinarily bepresent due to different degrees of thermal expansion.

In addition, the use of induction heating may eliminate wear-outmechanisms associated with the exposure of certain type of substrates14, such as plastic laminates, to an excessive number of heat cycles.These wear-out mechanisms may limit the number of permitted reworks ofthe solder joints 16. In addition, a reflow oven and associatedequipment is not required to join the chip 12 with the substrate 14.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIGS. 1-5 and in accordance with an alternative embodiment,the induction heater 48 may be carried by the pick-up head 42 of thepick-and-place machine 40 instead of being stationed proximate to thesubstrates 14. As the pick-up head 42 is moved by the robotic arm 41,the induction heater 48 moves along with the pick-up head 42. Theelectrical connection of the induction heater 48 with the power supply46 is configured to accommodate the mobility of the induction heater 48.If the induction heater 48 includes a coil, the coil may be wrappedabout the portion of the pick-up head 42 that carries the chip 12 sothat the chip 12 is at least partially surrounded by the coil and iscoupled with the time-varying magnetic from the coil to provide heatingby electromagnetic induction.

The induction heater 48 may be operated as discussed above to liquefythe solder bumps 50 during the time required for the pick-up head 42 tocomplete the motion over the motion path 43 from the storage location 44to the substrate 14. This arrangement of components may enhance theflexibility in the timing relative to contact with the substrate pads 54for the use of the induction heater 48 to change the phase of the solderbumps 50 from solid to liquid. The solder bumps 50 may be liquefied overany segment or portion of the motion path 43. For example, the solderbumps 50 may be heated by the induction heater 48 to provide the liquidphase immediately after the chip 12 is picked at the storage location 44by the pick-up head 42.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. Thechip may be integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either (a) anintermediate product, such as a motherboard, or (b) an end product. Theend product can be any product that includes integrated circuit chips,ranging from toys and other low-end applications to advanced computerproducts having a display, a keyboard or other input device, and acentral processor.

A feature “connected” or “coupled” to or with another element may bedirectly connected or coupled to the other element or, instead, one ormore intervening elements may be present. A feature may be “directlyconnected” or “directly coupled” to another element if interveningelements are absent. A feature may be “indirectly connected” or“indirectly coupled” to another element if at least one interveningelement is present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. An apparatus for joining a chip with a substrate,the apparatus comprising: a pick-and-place machine including a pick-uphead to for releasably holding the chip and a robotic arm for moving thepick-up head in a motion path; an induction heater including a pluralityof coil turns located in the motion path directly over the substrate,the coil turns having an inner diameter defining an interior surface;and a power supply coupled with the induction heater, the power supplyconfigured to supply electrical power with a time-varying alternatingcurrent to the coil turns of the induction heater for heating solderbumps on the chip by electromagnetic induction while the chip isreleasably held by the pick-up head, wherein the pick-up head moves thechip axially through the interior surface of the coil turns and towardthe substrate.
 2. The apparatus of claim 1 wherein the pick-and-placemachine continuously moves the pick-up head in the motion path towardthe substrate as the induction heater heats the solder bumps.
 3. Theapparatus of claim 1 further comprising: a controller coupled with thepower supply, the controller including at least one processor and amemory including program code that, when executed by the at least oneprocessor, causes the power supply to supply the electrical power withthe time-varying alternating current to the coil turns of the inductionheater.
 4. The apparatus of claim 3 wherein the controller is coupledwith the pick-and-place machine, and the program code, when executed bythe at least one processor, cause the robotic arm to move the pick-uphead in the motion path in which the chip is moved axially through theinterior surface of the coil turns.
 5. The apparatus of claim 4 furthercomprising: a conveyor, wherein the controller is coupled with theconveyor, and the program code, when executed by the at least oneprocessor, cause the conveyor to move the substrate to a location atwhich the coils turns are directly over the substrate.
 6. The apparatusof claim 1 further comprising: a conveyor for moving the substrate intoa location proximate to the induction heater.
 7. The apparatus of claim1 wherein the robotic arm is a programmable mechanical arm with aplurality of joints and a plurality of links connected by the jointsthat allow rotational motion and/or translational displacement of thepick-up head.
 8. The apparatus of claim 1 wherein the induction heaterhas a fixed position in relation to the substrate as the pick-up headmoves in the motion path.